Apparently the same species of bird can have variations to their song according to their geographic location. So I suppose the young lady in the Banks of Sweet Primroses who wants to disappear to some lonesome valley would find the birds singing a different tune. The reference to the birds changing their voices reinforces the remoteness and seclusion she seeks. A similar verse appears in some versions of Peggy Gordon and I imagine that the birds singing differently to indicate remoteness is an attractive concept that's led to this being a floating verse.

LFF